It is known to produce interference between matter waves which are associated with cold atoms, in order to measure accelerations very accurately. Devices which implement such atomic interferences are for example accelerometers, gyrometers, gravimeters, gradiometers, etc.
In a simplified manner, a matter wave associated with a set of atoms which are initially in a ground state, most often of a hyperfine structure, is divided into two by a first interaction with a laser radiation. The duration of this interaction can be selected for example so that the two matter waves have substantially equal amplitudes. Such an interaction has a function equivalent to that of a beam splitter in optics, and is called a π/2 pulse in the terminology of a person skilled in the art.
Additional interactions are then produced between the atoms of each matter wave and subsequent laser pulses, according to a sequence of successive interactions which finally produces the interference between the matter waves. A detector is then placed in the interference field, in order to measure the number of atoms which are in one or other of the atomic states: the ground state or the higher energy state. Due to the interference, this number of atoms measured depends on a phase shift which has been progressively accumulated between the two matter waves since their separation by the π/2 pulse, and which contains the sought measurement information.
Each interaction between an atom and the laser radiation has two simultaneous effects: causing a transition of the atom between its two quantum states, in a direction which is opposite with respect to an absorption and an emission, and simultaneously a variation in the momentum of the atom which corresponds to the momentum of the radiation absorbed or emitted.
Usually, a Raman source is used in order to generate the laser radiation which produces each interaction with one of the atoms. Such a Raman source in fact produces two monochromatic radiation components, each of laser type, the respective wavelengths of which are selected in order to cause a two-photon interaction with one atom. These two monochromatic components form beams which are spatially superimposed parallel to the same axis of propagation, and which each propagate simultaneously in the two opposite directions along this common axis. The interaction between such composite laser radiation and the atoms comprises an absorption and an emission, so that the difference between the energies of the atomic states is equal to the difference between the respective frequencies of the two monochromatic components of the radiation multiplied by Planck's constant h. The variation in the momentum of the atom which undergoes the interaction is then equal to the sum of these frequencies, multiplied by h/C, where C is the velocity of propagation of the laser radiation. FIG. 1 illustrates such interactions, in the case of an absorption of energy in the left part of the figure, and an emission in the right part of the figure. The references used in this figure have the following meanings:                F: ground state of the atoms        E: fine structure state of the atoms, which has a higher energy than that of state F        G: difference in energy between states F and E        λ1, λ2: respective wavelengths of the two monochromatic components of the laser radiation which is produced by the Raman source        
The Raman source can have several known implementations.
According to one of these implementations, the Raman source comprises two separate laser sources which each produce one of the monochromatic components of the total laser radiation of the Raman source. The respective frequencies of the two laser sources are precisely adjusted in relation to each other by superimposing the two monochromatic components on a rapid photodiode. The photodiode then makes it possible to detect the beat of the wave superimposition, with the frequency of this beat corresponding to the difference between the frequencies of the two laser sources. It is thus possible to precisely tune this difference in frequencies to the energy difference G between the atomic levels F and E. But such a implementation of the Raman source is complex and bulky since it comprises two separate laser sources. For this reason, it is not suitable for producing devices for measurement by means of atom interferometry which are compact and weigh as little as possible. The weight constraint is even more significant for devices which are intended to be carried on board an aircraft such as a plane or helicopter, or on board a spacecraft such as a rocket, satellite or space probe.
According to another known implementation, the Raman source comprises only a single laser source, which is amplitude- or frequency-modulated, or both. Such a Raman source is described in particular in the article by O. Carraz et al. entitled “Compact and robust laser system for onboard atom interferometry”, Applied Physics B (2009) 97, pp. 405-411, and also in the thesis by O. Carraz entitled “Gravimètre atomique embarquable: Etude théorique et expérimentale de l'instrument”, and presented at the Paris Observatory on 19th December. The Raman source which is thus constituted is particularly compact and robust, such that it is suitable for producing measurement devices that are themselves less bulky and less heavy. But such a method of generating the laser radiation for Raman interactions—by modulation of a single laser source—simultaneously produces at least three monochromatic radiation components which have distinct respective frequencies. Now, only two of these frequencies are useful for measurements which are carried out by means of atom interferometry. The additional monochromatic component which results from the modulation of the laser source then produces a measurement bias, which in turn produces an error in the measurement result.